The amount of nitrogen oxides (NOx) in flue gases from combustion processes can be decreased by reducing these oxides. For this purpose, one can use compounds containing ammonium nitrogen, such as ammonia, urea, ammonium carbamate, ammonium carbonate, ammonium bicarbonate, ammonium formate, ammonium oxalate or ammonium hydroxide.
In nitrogen oxide removal, one significant alternative is to use selective catalytic reduction (SCR techniques), in which nitrogen oxides are reduced to nitrogen and water by means of a catalyst and a reducing agent. Especially ammonia and urea have been used as reducer. The reduction temperature has usually been 265 to 425° C., typically 350 to 400° C. For example V2O5—TiO2 catalyst has been used as catalyst.
In SCR techniques for vehicles, ammonia and ammonia water cause problems due to operational safety reasons. Consequently, it is preferred to use urea, for instance, which is almost risk-free. Another reason for the advantageous use of urea is that it contains twice as much nitrogen per mol as the ammonium salts mentioned above. In fact, an aqueous solution of urea is a very expedient nitrogen oxide remover. Urea is dissolved to an approx. 32.5% of weight solution, whereby solubility is adequate and temperature stability lowest (approx. −11° C.).
However, in Northern Europe and colder regions in general, the problem is that the outdoor temperature is often below −11° C., so that there is an evident risk of freezing of the urea solution. The freezing problems can be reduced by installing various temperature trackers and resistors by which the freezing is mainly prevented or by which a frozen urea solution is dissolved back to liquid form.
Literature mentions the use of ethanol [F. F. Mao et al. “Urea-ethanol-water solution for diesel NOx control Using Urea”, 6th Diesel Engine Emissions Reduction (DEER) Workshop August 20-24] and propylene glycol [Lambert et al. “Application of Organic Freeze-Point Depressant in Aqueous Urea Solution: Effect of Nox Reduction”, SAE2003-01-0775] as anti-freezing agents of urea water solution in the reduction of nitrogen oxides of exhaust gases in diesel combustion process. Publication No. U.S. Pat. No. 6,387,336 discloses that it is possible to decrease the freezing point of urea water solution with additives, such as ammonium formate (column 1 lines 46-60). According to the publication these additives are, however, especially corrosive, so their use is problematic. As a further disadvantage is considered that due to the water contained in the composition, the temperature of the exhaust gas will decrease weakening the activity of the catalyst. Therefore, ammonia is used as NOx reducing agent in accordance with the publication.
Also selective non-catalytic reduction (SNCR techniques) is used. For example in publication No. U.S. Pat. No. 5,116,584 especially the SNCR techniques is described. Usually a relatively high temperature, typically 900 to 1000° C., is used in SNCR techniques. Publication No. U.S. Pat. No. 3,900,554, however, discloses a process, in which ammonia, ammonium formate, ammonium oxalate or ammonium carbonate is used as reducing agent and in which the temperature of exhaust gases is 704 to 1093° C. Publication No. U.S. Pat. No. 4,873,066, in turn, discloses a SNCR process, in which tri-ammonium citrate or ammounium formate is used as reducing agent, so that the temperature of exhaust gases can be relatively low, below 649° C. 482° C. is mentioned as the lowest temperature.